Or HaChaim on Deuteronomy 4:3/אור החיים על דברים ד׳:ג׳

1 עיניכם הראות, "you have seen with your own eyes, etc." Moses meant that the Israelites had first- hand experience of the severity with which G'd punishes idol worship. G'd relates to that sin more harshly than to any other sin. He punishes even the people who have not actually served the idol with any of their limbs or their organs but have merely entertained idolatrous thoughts. G'd destroyed people who intended to worship Baal Pe-or. Kidushin 39 cites Ezekiel 14,5 as proof that G'd punishes the mere intention to indulge in idol worship.